A visual tracking system has been employed in the measurement ofdeck displacemenls at the centre ofthe l4lom span ofthe Humber Bridge. The transputer-based system was developed for applications such as the monitoring or testing of large physioal structures, *here the actual displacements may not easily be otherwise determined. The system employs parallel processing techniques to track predictively the motion of multiplg independent objects at video frame mte. Results obtained using the system in amonitoring programme ofthe bridge are presented and discussed. vEeo v3.0 3/'t992 .

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SUMMARYAmbient vibration tests were conducted on a 56 metre high concrete gravity dam to measure its modal properties for validating a "nite element model of the dam}reservoir}foundation system. Excitation was provided by wind, by reservoir water cascading down the spillweir, and by the force of water released through outlet-pipes. Vibrations of the dam were measured using accelerometers, and 3-hour data records were acquired from each location. Data were processed by testing for stationarity and rejecting nonstationary portions before Fourier analysis. Power spectra with low variance were generated from which natural frequencies of the dam were identi"ed clearly and modal damping factors estimated. Modal analysis of the frequency response spectra yielded mode shapes for the six lowest lateral modes of vibration of the dam. The "nite element model for the dam was analysed using EACD-3D, and the computed mode shapes and natural frequencies compared well with the measured results. The study demonstrates that ambient vibration testing can o!er a viable alternative to forced vibration testing when only the modal properties of a dam are required.